There's just so many out-there that's it's difficult to decide..
Phoenix / Elixir has gotten my interest, but I don't know about authentication..
Ideas?
I vote Elixir and Phoenix.
For me the biggest hurdle was and still is wrapping my head around a language that is pretty seriously functional. I've been a huge fan of functional programming for a while, but I always worked in 'general-purpose' languages like Python, JavaScript and Ruby. While it can be frustrating, I feel this process of actually working functionally with less 'escape hatches' will be worthwhile even if I end up spending most of my time in the JS world.
But for anyone who is interested in functional programming, but never bothered to learn Haskell or the like, or anyone who feels Rails is not cutting it anymore, for whatever reason, I can highly recommend Elixir/Phoenix.
Furthermore, for me OTP is just awesome. I do not have the experience to make statements about whether OTP and the process-based approach to concurrency is better than alternatives, or even good in itself. But there's something about the relative simplicity, the idea of just processes communicating and some standardization of this, that tickles me in the same way functional programming tickled me when I was first introduced to it.
The idea that any approach to 'asynchronous programming' in Elixir really just boils down to a bag of functions running in one or multiple 'processes', and that this is the exact way I treat, say, bash scripting, is extremely exciting to me for some reason.